Anemia means that the blood cannot carry
as much oxygen as it should, either because of a reduced number of red
blood cells or a reduced amount of hemoglobin in those cells.
Normal blood is 40 to 45 percent red cells and 55 to 60 percent
plasma. On the average there are 12.5 to 14 grams of hemoglobin
per 100 milliliters of blood. The normal red-cell count is 4.5
to 5.5 million cells per cubic millimeter. All these values tend
to be 10 percent lower in women.
In anemia there is either a reduced
amount of hemoglobin in each of the red blood cells or else the total
number of red cells is less than usual. In either case the
oxygen-carrying capacity of the blood is reduced and the tissues
suffer. Strangely, when there is an excess of red blood cells,
the tissues may also suffer for lack of oxygen. In this case the
difficulty is that with a high population of red cells the blood
becomes syrupy and moves so slowly that it cannot deliver its oxygen
Anemia may result from improper
formation of red blood cells by the bone marrow. A small amount
of vitamin B12 is necessary for the cells to mature, and iron combined
with protein is needed so that each cell may receive its hemoglobin.
Anemias resulting from failure of this system are called anemias of
production. Other anemias occur when red blood cells are
destroyed prematurely. These are called hemolytic anemias.
When red blood cells are lost because of bleeding, the resulting
anemia is called anemia of hemorrhage, or secondary anemia.
Finally, anemia due to damage of the bone marrow (where the red blood
cells are produced) is called aplastic anemia.